1. Field of the Invention
The present invention relates to a transmitter used for a communication system having a high peak-to-average power ratio (PAPR), and more particularly, to an amplifier and an amplifying method. Known examples of a system having a high PAPR include an OFDM (orthogonal frequency division multiplexing) communication system.
2. Description of the Related Art
FIG. 1 shows a typical OFDM transmitter. As shown in FIG. 1, the OFDM transmitter includes: a serial-to-parallel converter (S/P) 1 for performing serial-to-parallel conversion on an input bit stream (source of data to be transmitted); an inverse fast Fourier processor (IFFT) 2 having a size of M; a parallel-to-serial converter (P/S) 3; an oversampling block (interpolator) (OS) 4; a digital-to-analog converter (DAC) 5; an output low-pass filter (LPF) 6; an RF up converter (RF) 7; and a high-power amplifier (HPA) 8 that operates in class A or class A-B.
The OFDM transmitter shown in FIG. 1 generates an OFDM signal having a high PAPR level. In order to amplify the high-PAPR OFDM signal, it is necessary to employ a high input back-off level linear amplifier that operates in class A or class A-B. However, the amplifier of this type is low in power efficiency.
The simplest approach to reducing a PAPR of the OFDM signal is to clip (cut out) high amplitude peaks thereof. A variety of clipping techniques have been proposed. In some of the techniques, outputs of an inverse fast Fourier transform (IFFT) are clipped before interpolation (oversampling (OS)). However, the signal must be interpolated before digital-to-analogue conversion, which may cause peak re-growth. To avoid the problem of peak re-growth, the signal may be clipped after interpolation. However, such the scheme causes extremely significant out-of-band power.
A single clipping method may be employed as one of approaches for reducing the out-of-band power to a predetermined level. FIG. 2 shows a configuration example of an OFDM transmitter to which a PAPR reducing scheme using the single clipping method is applied.
The OFDM transmitter shown in FIG. 2 is provided with: an IFFT 9 in place of the IFFT 2 shown in FIG. 1; a limiter 10 to which an output (OFDM signal) from the P/S 3 is inputted; and an LPF 11 to which an output from a limiter 10 is inputted, the LPF 11 being connected to the DAC 5.
The input vector is first converted from a frequency domain to a time domain by using an oversize IFFT 9. In a case where an oversampling factor of the IFFT 9 is K, the input vector is extended by adding M*(K−1) zeros in the middle of the vector. As a result, trigonometric interpolation is performed in the time domain. The oversampled or interpolated signal is then clipped by the limiter 10. In the limiter 10, hard-limiting is applied to the amplitude of the complex values x at the IFFT output.
[Expression 1]
                                          INPUT            ⁢                          :                        ⁢                                                  ⁢            x                    =                      ρ            ·                          exp              ⁡                              (                                  j                  ·                  ϕ                                )                                                    ⁢                                  ⁢                                  ⁢                              OUTPUT            ⁢                          :                        ⁢                                                  ⁢                          y              ⁡                              (                x                )                                              =                      {                                                            x                                                                                            for                      ⁢                                                                                          ⁢                      ρ                                        <                    A                                                                                                                    A                    ·                                          exp                      ⁡                                              (                                                  j                          ·                          ϕ                                                )                                                                                                                                                        for                      ⁢                                                                                          ⁢                      ρ                                        >                    A                                                                                                          EXPRESSION        ⁢                                  ⁢                  (          1          )                    
In Expression (1), A is a clipping level (CR). Note that both x and y are complex values. The clipping ratio CR is defined as a ratio of the clipping level A to the clipping level to a mean power of an unclipped baseband signal. After the clipping, frequency domain filtering is performed by the LPF 11, thereby performing waveform shaping and reducing the out-of-band power.
FIG. 3 shows a cumulative distribution function (CCDF) for clipping scheme shown in FIG. 2 in terms of different clipping levels (CR). In simulations shown in FIG. 3, a modulation scheme is QPSK (quadrature phase shift keying), where M=64 and K=2. A plot that corresponds to an OFDM transmission scheme (OFDM transmitter) shown in FIG. 1 is indicated as “OFDM” in FIG. 3. The filtering performed by the LPF 11 exhibits some peak re-growth (see FIG. 3). More compact CCDF is preferable in order to decrease a dynamic range of the HPA 8 and increase power efficiency of the HPA 8.
In addition to the OFDM transmitters shown in FIGS. 1 and 2, prior art with respect to the present invention includes techniques described in the following related art documents.
[Patent document 1] JP 2001-189630 B
[Patent document 2] JP 2002-368716 B